Substitute energies for oil are now being developed all around the world. Among them, a process to produce industrial gas rich in methane (CH.sub.4), hydrogen (H.sub.2) and carbon monooxide (CO) by gasifying coal, or a process to produce synthetic natural gas by methanation of these gases are regarded as promising. Especially a gasifying method utilizing a fluidized bed is superior to other methods in scale up and operatability and has possibility as a practical gasifier.
As a form of gasification, it is most reasonable from the viewpoint of efficient use of heat to carry out carbonization and partial combustion of char produced in carbonization at the same time. In this partial combustion system utilizing a fluidized bed, it is most important for improving gasification efficiency to effectively treat fine dusts and prevent calorie decrease caused by excessive CO.sub.2 (carbon dioxide).
Coal particles pulverized into less than several mm diameter are generally used in the fluidized bed gasifier and the fine particles hve a wide particle distribution size as shown in FIG. 1. Particularly fine dusts below several hundred microns entrained with gas from the fluidized bed gasifier cause plugging in the gas purification section. In addition, not only is it intricate to transporting, after collecting by a cyclone separator, but also it brings about carbon loss because it is not fully gasified, and, as a whole, the gasification efficiency is lowered.
Therefore, fine coal is separated and removed before it is supplied to the gasifier and it is used for other purposes such as a fuel for generating steam.
It is also practiced, in order to improve gasification efficiency to put back the fine char collected from product gas by a cyclone separator and to carbonize (thermal cracking) it with raw material.
However, in the first method, various pollution problems arise such as fly ash being dispersed into the air and air pollution caused by harmful matters in the exhausted gas. As a countermeasure to a large scale pollution, a prevention system is required and as a result the gasification cost will be raised.
In a second method of putting back the collected char into the gasifier, ash produced by combustion is again exhausted and dispersed, and as a result, it circulates between the gasifier and the cyclone separator and is retained within the gasification equipment. In this case, not only the ash but also the collected char in one path are contained in the fine dusts circulating in the gasification equipment. So, if they are exhausted out of the equipment, it brings about carbon loss. On the other hand, there is a method for gasifying only fine pulverized coal in an air stream, but this method is adopted for producing raw material gas consisting mainly of H.sub.2 and CO and is not adequate for producing high calorific gas.